Is jumbo ethernet MTU possible with Hamachi Gigabit ethernet driver?

[John Weidman <johnw@believe.com>]

I'm trying to get some Gigabit ethernet cards that use the Packet
Engines Hamachi GNIC-II chip to use a large mtu to attempt to get a
throughput of close to the 1Gb rating of the card.  This is on a Compact
PCI Alpha system.  I'm trying to use an MTU in the 8000 to 9000 range
and so far have not been able to get these MTUs to work.

I have changed the PKT_BUF_SZ and MAX_FRAME_SIZE constants in hamachi.c
and ETH_DATA_LEN and ETH_FRAME_LEN in if_ether.h.  I can use ifconfig to
change the MTU above 1500 on one side of a connection but as soon as I
raise the MTU on both sides to greater than 1500 the link dies.  I can
change the MTU with ifconfig back to 1500 and the link will resume
operation.  We are currently somewhat married to the 2.2.14 kernel.

I read that some ethernet drivers will support jumbo MTUs.  There
appears to be something in the hamachi driver or the kernel that I've
missed.  Perhaps this only works with a later version kernel or the
hamachi driver needs more changes?  Any help would be appreciated.

John Weidman
johnw@believe.com